Serious enjoyment cycling by a French city (7)
I believe the answer is:
unfunny
'serious' is the definition.
(thesaurus)
'enjoyment cycling by a french city' is the wordplay.
I cannot really see how this works, but
'enjoyment' could be 'fun' (enjoyable activities) and 'fun' is found within the answer.
'city' could be 'ny' (short for New York) and 'ny' is located in the answer.
'a french' could be 'un' ('a' in French) and 'un' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for unfunny that I've seen is " Not amusing".)